Aviation gasoline is a product which is elaborated with care and subject to severe regulations, which go hand-in-hand with any aeronautical application. Thus, the preparation in a refinery of aviation gasoline is carried out with bases characterized by both a narrow distillation interval and high octane numbers. These bases generally consist of alkylates, reformates and/or isopentane cuts, the latter being used in a low concentration because of their high volatility.
But prior to their marketing, aviation gasoline should satisfy other specific physico-chemical characteristics, defined by international specifications. Thus aviation gasolines should have                a low vapor pressure (less than 490 mbars) in order to avoid vaporization problems (vapor lock) or risks of icing increased by the low temperatures encountered in altitude;        a final distillation point of less 170° C., in order to limit formations of deposits and their harmful consequences (power losses, deteriorated cooling);        good cold strength (absence of crystals down to −58° C.) in order to prevent any blocking of the filters and conduits in altitude;        a low sulfur content, presently limited to 0.05% by weight, all these characteristics being grouped in the ASTM D 910-07 standard in effect at the filing date of the present application.        
Aviation gasolines are used on engines having good performances and frequently operating under a high load, i.e. under conditions close to pinking. It is therefore required that this type of fuel have very good resistance to self-ignition.
For aviation gasoline, the motor octane number or MON is determined relatively to the operation with a slightly lean mixture (cruising speed). With the purpose of guaranteeing this high octane requirement, the refiner generally proceeds at the stage of the making of aviation gasoline, with the addition of a organolead compound, and more particularly tetraethyl lead (TEL). Now, like for gasolines for land vehicles, government authorities tend to lower the lead content, or even to suppress this additive, because of its noxious character for the environment and health. Thus, firstly, reduction and then elimination of lead from the composition of aviation gasoline become a goal to be attained in the short and then in the medium term.
The use of additives as a replacement for lead compounds in order to improve the octane number, is well-known for land vehicle gasolines, such as in patents EP 474 342, GB 2,114,596, U.S. Pat. No. 5,032,144 or U.S. Pat. No. 4,647,292. Insofar aviation gasoline is subject to much more severe regulations than gasoline for land vehicles, for reasons of reliability and safety of operation during the taking-off phase and in altitude, it is not obvious to use these additives for aviation gasoline, for which the constraints are much stricter than those required for automotive gasolines.
As regards aviation gasoline, in order to replace the lead additives, solutions have been proposed in the literature, such as by adding other additives or components, such as aromatic amines, for example aniline, alkylaniline, methylcyclopentadienyl manganese tricarbonyl (MMT).
EP 540 297 and EP 609 089 propose adding methylcyclopentadienyl manganese tricarbonyl (MMT) to aviation gasoline as a substitute for lead.
In EP 609 089 lead-free aviation gasolines are described, based on alkylates, in which an ether is added, such as ETBE or MTBE, as well 0.4 to 0.5 g/gallon of manganese in the form of the cyclopentadienyl manganese tricarbonyl compound. The problem of the invention was not discussed since the described gasoline is not a base of the AVGAS type: it comprises alkylates, but does not comprise reformates, or isomerates.
In EP 697 033, a lead-free aviation gasoline is described consisting of isopentane, alkylate, toluene, with a MON comprised between 90 and 93, additived with 4 to 20% of an aromatic amine in order to obtain a gasoline with an MON greater than or equal to 98.
EP 910 617 describes a mixture of aromatic amine and alkyltertiobutyl ether, and optionally of a manganese compound which is added to the alkylate with a wide boiling range, forming aviation gasoline.
In WO 02/22766 a lead-free aviation gasoline is described, comprising a hydrocarbon compound which may be triptane associated with at least one liquid, saturated, aliphatic hydrocarbon compound (4 to 10° C.) and further comprising an alkyl ether (the thereby obtained base is not an AVGAS base).
Even if these additives lead to an increase in the octane number, this increase sometimes remains insufficient. Further, addition of these additives does not always meet the other requirements specific to aviation gasoline, such as heating value, vapor pressure and distillation range.
Moreover, these different additives are not always compatible with being respectful to the environment. For example, aromatic amines widely used for improving the octane number are all classified as toxic substances, in the case of absorption by inhalation or ingestion and especially on the skin. Methycyclopentadienyl manganese tricarbonyl (MMT) is as for it indexed by EPA as an air pollutant, which may represent potential risks for humans. Oxygenated compounds are presently not allowed in aviation gasoline compositions.
This is why other additive-free technical solutions have been proposed:
EP 948 584 proposes an aviation gasoline with a MON above 98 which contains at least 30% by volume of triptane and/or of 2,2,3-trimethylpentane.
EP 1 359 207 describes a lead-free automotive gasoline composition with an MON between 80 and 98 containing from 5 to 25% by volume of triptane and/or of 2,2,3-trimethylpentane, from 5 to 15% of olefins, from 15 to 35% of aromatics and 40 to 65% of C4-C12 paraffins.
EP 1 224 247 describes a lead-free gasoline which may notably be used as an aviation gasoline with an MON of at least 80, with a RON comprised between 90 and 115 containing a C8-C12 alkane with at least 4 methyl and/or ethyl branches. Preferably, the preferred gasoline further comprises triptane and/or 2,2,3-trimethylpentane.
Now the addition in a significant amount of compounds such as triptane and/or 2,2,3-trimethylpentane or alkanes with at least 4 branches which are a very small minority in usual oil bases stemming from refineries, and not only requires costly separation and purification processes for synthesizing these compounds but also generates for the refiner additional technical difficulties for storage and logistics for these compounds generating costs above those generally obtained for formulating conventional aviation gasoline from a mixture of hydrocarbon bases usually available in refineries.
WO 04/044106 describes lead-free aviation gases compositions with an MON ranging from 92 to 98 and containing from 10 to 90% by volume of at least one trimethylpentane and at least one C4-C5 paraffin. An example of a gasoline composition according to the invention with an MON of 95 comprises 59% isooctane, 8% toluene, 16% isopentane, 24% by volume of alkylate and 16% of alkylate fraction other than isooctane.
DE 197 44 109 describes a lead-free gasoline composition for 2- and 4-stroke engines comprising 70-85% by volume of C8 isoparaffins, 17-19% by volume of C5 isoparaffins, 2-4% of C6 isoparaffins and preferably not more than 0.5% of aromatics and 0.1% of benzene. This reference does not specify whether this gasoline is suitable as an aviation gasoline.